(1) Field of the Invention
This invention relates generally to bandgap voltage reference generators and relates more specifically to a startup circuits for bandgap voltage reference generators.
(2) Description of the Prior Art
Typical bandgap voltage reference generator circuits are self-biased and have two stable bias-points, a desired operational bias point for which the circuit has been designed for and an undesired bias-point where the bandgap output voltage reference remains at 0 V. A dedicated startup circuit is required to ensure that the bandgap voltage reference generator is brought to an operating range where it can obtain the desired bias-point.
Current practice is to monitor a voltage inside the bandgap voltage reference generator and to initiate a current flow into the bandgap reference voltage generator circuit as long as the voltage monitored does not have the operation condition required. The disadvantage of monitoring a voltage is that the voltage monitoring is performed by a voltage comparator, which requires a specific voltage for its operation and a reference voltage.
It is a challenge for the designers of startup circuits for bandgap voltage reference generators to design a circuit wherein the startup condition is process independent because a threshold voltage of NMOS or PMOS devices is used as reference and wherein no reference voltage is required for the startup.
There are known patents or patent publications dealing with startup circuits for bandgap voltage reference generators.
(U.S. Pat. No. 6,972,550 to Hong proposes a bandgap voltage reference generator including a bandgap voltage reference circuit and a fast startup circuit. The fast start-up circuit, which is cost-efficient and saves power consumption, can rapidly start up the bandgap reference voltage circuit coupled thereto.
U.S. Patent Publication (US 2007/0241735 to Rabeyrin et al.) discloses a power efficient startup circuit for activating a bandgap reference circuit. The startup circuit uses a voltage supply having a voltage level to initiate the flow of a startup current used to activate the bandgap reference circuit.
U.S. Patent Publication (US 2008/0018317 to Chen et al.) describes a bandgap reference circuit having a low sensitivity to temperature and supplied voltage installs a compensation circuit on a bandgap reference circuit to substitute a prior art that uses a resistor to match the circuit startup purpose and solve the problem of startup error caused by the manufacturing error.
Furthermore there are two more patents dealing with startup circuits for bandgap voltage reference generators:
(U.S. Pat. No. 6,784,652 to Aude) describes a startup circuit for a bandgap voltage reference generator circuit. Monitoring an internal reference voltage of the bandgap voltage reference generator circuit, current flow for the bandgap circuit diodes is initiated following circuit startup, e.g., initial application or DC power
(U.S. Pat. No. 6,784,652 to Yu) discloses a circuit including a band-gap reference circuit and a start-up circuit coupled between an output and an input of the band-gap reference circuit. When the output of the band-gap reference circuit is below a start-up voltage threshold, the start-up circuit provides a first voltage at the input of the band-gap reference circuit, which, in turn, causes the band-gap reference circuit to produce a desired voltage at the output.